Siratification of France and England. 139 



ance, economic uses, or conspicuous fesils, may have a 

 colour assigned tliem^ especially if such Fall between, and 

 not so as to subdivide thick masses, selected for separate 

 soils, or fi^enera on the principles 4ibove. 



In coaf-districts or others, where grit or limestone alter- 

 nate with argillaceous strata, it will be found right perhaps, 

 to select all ihe thick masses of rock, which are sufficiently 

 separated by clay-shists and other argillaceous strata, to 

 have diflcrent colours assigned thcni, either using some 

 general colour for the clays formed on the surface by all the 

 argillaceous strata, or assigning a colour to each of these 

 aroillaceous assemblages, according to the number of the 

 strata and thicknesses of them, so that the map be not over 

 crowded. The transitions frc^ni light to heavy or dry to wet 

 latul, made by the above two classes of strata, are the best 

 deHu(;d characters that we meet w ith in mineralogical sur- 

 veying ; and arc indeed almost the only ones by w hich the 

 fanners and occupiers of the lands can give any assistance 

 by their information, except while pacing over every part 

 of the surface with the surveyor, in answer to his questions, 

 as they may arise. 



The plan which M. Cuvier and M. Brogniart seem to 

 have followed, in assigning colours to their soils (p. 41), and 

 dividing some parts of the Paris strata into their ten forma- 

 tions or soils, seems to me much inferior to the above ; and 

 in particular their sea sands, freestone without shells, and 

 fresh-water soils, appear calculated to lead to no useful re^ 

 suit, but rarher to mislead. The more popular and obvious 

 the divisions of the strata are made on mineralogical maps 

 and sections, the sooner will the geological facts thev con-* 

 vey be understood and rendered useful ; for after all that 

 can be said, they must, in the present state of our know» 

 ledge at least, be but arbitrary classes or divisions, exhibitiuc'' 

 the order of super-position, principally. 



llie freedom of the remarks v.hich I have ventured to 

 make on the opinions and writings o? geological observers 

 of such high celebrity, will J trust in candour be ascribed 

 to their true motives, — a desire to render justice to a valuable 

 friend and to our c'oimtry, and to advance and perfect the 

 science; which of all others seems to me to claim our seri- 

 ous cultivation and attention. An apology may perhaps be 

 necessary to your other correspondents and readers for the 

 great number of your valuable pages which I hav^e occupied. 

 I am, sir. Your obedient servant, 



John Fa uey. 



FeVriiarv !?.rh, IRIO, 

 l^a. 12, Upper Crown-street, Westminster, 



